 In the last video we talked about expressing rates in terms of how fast a reactant disappears or a product is produced. Here we're going to go further and produce an equation that predicts what the rate of reaction will be for a given reactant concentration. We're going to use experimental data to do this. This equation is called a rate law. There's no way of determining a rate law just by looking at a chemical equation. It has to be experimentally determined and this is done by studying how the rate changes as you change the concentration of a reactant just as we discussed in the last video. So imagine I have a reaction and one of its reactants is called A. I vary the concentration of A and I collect some data on the initial rate for each concentration keeping everything else constant. I then make a new graph using axes like this and I look at the shape of the graph because this can tell me some important information about the reaction. For instance if there's a linear relationship between the rate and the concentration this means that the rate is directly proportional to the concentration of that reactant and we call that a first order dependence. We say that the reaction is first order with respect to A. If the rate has a parabolic dependence on concentration you get a curve like this then we know that the rate is proportional to the square of the reactant concentration and we say that the reaction is second order with respect to A. It's also possible to have higher orders, third order say, or fractional orders like a half or a two-thirds but we're unlikely to meet those in this course. Another possibility is that the rate doesn't depend on this reactant at all. No matter how you change the reactant's concentration the rate doesn't change. This is called a zero or zeroth order dependence. Note that it doesn't mean that the reactant's not taking part in the reaction just that the rate of the reaction is not dependent on the concentration of this reactant. We can turn these proportionalities into proper equations that describe how the rate of the reaction depends on the concentration of its reactants and this equation is called the rate law for that reaction. Each reaction has a different rate law and each reactions law has to be determined by running rate experiments.